System for applying microwave energy to a lossy cylindrical object

ABSTRACT

In a system for heating lossy dielectric cylindrical objects, for example in pre-cooking Vienna sausage, a wave guide of rectangular cross section is excited in the TE10 mode to generate an electric field extending perpendicularly between the broadwalls of the wave guide and having a maximum intensity in a plane midway between the sidewalls extending in the direction of power flow. The cylindrical object is then passed through the opposing broadwalls with the axis of the object extending generally in the plane of maximum field intensity, forming an acute angle of incidence in the range of 15* to 80* with the axis of the guide. The angle depends on the dielectric constant and loss factor of the object, and it is set to reduce reflection back to the microwave source yet increases heating of the object and provides a more uniform heating of its surface. In a preferred embodiment the wave guide is folded back upon itself to provide a meander system with the object making three passes through each wave guide applicator.

United States Patent J ohnson [21] Appl. No.1 52,505

[52] US. Cl ..2l9/l0.55, 219/1061 [51] Int. Cl.

[58] Field ofSearch ..219/10.55, 10.61

[56] References Cited UNITED STATES PATENTS 3,457,385 7/1969 Cumming ..219/10.55 3,397,296 8/1968 Curran ..2l9/10.55

2,650,291 8/1953 Kinn ..2l9/10.55

2,483,933 10/1949 Revercomb et al. .....219/10.55

2,640,142 5/1953 Kinn ..2l9/10.55

3,235,702 2/1966 Timmermans ct al.. 19/1055 3,463,894 8/1969 Bleackley 219/1055 X 3,427,171 2/1969 Jeppson ..219/10 55 [451 May 23, 1972 2,958,830 11/1960 Bird et a1. ..2l9/10.55 X

Primary Examiner-J. V. Truhe Assistant Examiner-Hugh D. Jaeger Att0rney-Dawson, Tilton, Fallon & Lungmus [57] ABSTRACT In a system for heating lossy dielectric cylindrical objects, for example in pre-cooking' Vienna sausage, a wave guide of rectangular cross section is excited in the TE mode to generate an electric field extending perpendicularly between the broadwalls of the wave guide and having a maximum intensity in a plane midway between the sidewalls extending in the direction of power flow. The cylindrical object is then passed through the opposing broadwalls with the axis of the object extending generally in the plane of maximum field intensity, forming an acute angle of incidence in the range of 15 to 80 with the axis of the guide. The angle depends on the dielectric constant and loss factor of the object, and it is set to reduce reflection back to the microwave source yet increases heating of the object and provides a more uniform heating of its surface. In a preferred embodiment the wave guide is 1 folded back upon itself to provide a meander system with the object making three passes through each wave guide applicator.

13 Claims, 7 Drawing Figures Pafented May 23, 1972 I: 511cc Ls-Shcot I 15 7 J5 i I {0 I 2 I l|| a? Z] 3/ Z5 Z0 8 27 H W 2 w Z 1! v I Z Z. M my l1 HI -39 In d: nix) fi flyzy/zrzsarc SYSTEM FOR APPLYING MICROWAVE ENERGY TO A LOSSY CYLINDRICAL OBJECT BACKGROUND AND SUMMARY The present invention relates to a system for applying microwave energy to a lossy cylindrical object for example, in the pre-cooking of Vienna sausage prior to the slicing and canning operations.

Systems are known for heating objects, including food products, with microwave energy. These developments include: a batch type oven in which food material is placed in a multimode cavity, continuously operated microwave ovens wherein the food is passed through a multimode cavity, and serpentine configurations of wave guides of rectangular cross section in which sheet material (such as wall board) is passed through aligned slots in opposing broadwalls of a number of wave guide sections folded so that all of the slots are aligned. In the latter application, the direction of movement of the wall board is perpendicular to the plane of the broadwall through which it enters.

A co-pending, co-owned application of mine entitled CON- TINUOUS MICROWAVE HEATING AND COOKING SYSTEM, Ser. No. 816,722, Filed Apr. 16, 1969, now abandoned, discloses a rectangular wave guide applicator excited in the TE mode wherein the material being treated is conveyed through the applicator along the direction of power flow. The wave guide is formed in a general U-shape with the product entering at one elbow of the U and exiting through the same broadwall at the other elbow. The symbol TE refers to the Transverse Electric Field Vector; and TM refers to the Transverse Magnetic Field Vector.

In the present invention, a wave guide of rectangular cross section is excited to generate an electrical field in the TE mode wherein the electric field vector extends perpendicularly between the broadwalls of the wave guide. In the direction transverse of the elongation of the wave guide and from side wall to side wall, the electric field has an intensity profile which is zero at the side wall and increases to a maximum at the center of the guide. Thus, a plane which extends in the direction of power flow (i.e. the direction of elongation of the wave guide) and located midway between the side walls and perpendicular to the broadwalls may be referred to as the plane of maximum field intensity.

The object is passed through the broadwalls with the axis of the object generally in the plane of maximum field intensity and forming an angle of incidence, a, with the axis of the wave guide in the range of to 80. The optimum angle of incidence depends upon the dielectric constant and loss properties of the object; and the reasons for setting this angle are to reduce the reflected power and enhance a uniform coupling of microwave energy to the surface of the cylindrical object. The angle a is best determined empirically.

In a preferred embodiment, the wave guide is folded to form a meander system with the microwave power passing across the path of the cylindrical object at least two, and preferably three, times with the axis of the object still remaining in the plane of maximum field intensity for all passes. The wave guide is terminated in a water load downstream of the heating area, and a common source of microwave power may excite more than one applicator so that a plurality of objects may be processed simultaneously.

Other features and advantages of the present invention will be apparent to persons skilled in the art from the following detailed description of a preferred embodiment accompanied by the attached drawing wherein identical reference numerals will refer to like parts in the various views.

The Drawing FIG. 1 is a perspective view of a number of wave guide applicators for heating lossy dielectric objects according to the present invention;

FIG. 2 is a cross section taken along the sight line 2-2 of FIG. 1;

FIG. 3 is a plan view of the system of FIG. 1;

FIG. 4 is a close-up illustration of the angle of incidence; and

FIGS. 5-7 are tutorial diagrams explaining the underlying scientific principle of the present invention.

DETAILED DESCRIPTION Turning first to FIG. 1, reference numeral 10 generally designates a source of microwave energy shown schematically in phantom. In the illustrated embodiment, there are eight separate individual applicator sections energized by the source 10, however, the individual applicators are grouped in pairs and each pair is energized from a common feed wave guide. The feed wave guides are designated respectively 11, 12, 13 and 14. Preferably there is a separate magnetron tube energizing each of the feed guides l l-14, although each magnetron tube may be oscillating at the same frequency.

The power level of the magnetrons as well as the selected frequency will, of course, depend upon the results desired to be accomplished (that is heating or cooking), the dielectric loss factor of the product, the size of the product, etc. Persons skilled in the art of microwave cooking or heating will readily appreciate that such factors as these are best determined empirically and that no individual operating parameter is extremely critical.

Each of the feed wave guides 11-14, of course, are made of conducting material such as aluminum; and each is rectangular in cross section. The dimensions of the broadwalls of the feed wave guides is denoted a, and the dimension of the side walls is denoted by b, as seen in FIGS. 1 and 4.

The feed wave guides 11-14 are each formed into an elbow as at 15, 16, 17 and 18 respectively. Each ofthe elbows 15-18, in turn, feeds a T to divide the incident microwave energy into two separate rectangular wave guides, each feeding an applicator section. The T"' associated with the elbow 15 is generally designated by reference numeral 20, and it divides the incident microwave power to feed two rectangular wave guides 21 and 22.

The Ts fed by the wave guides 12, 13 and 14 are similar to the T 20, and need not be described in further detail for a complete understanding of the present invention. Similarly, each of the Ts associated with the wave guides 12-14 feed separate rectangular wave guides, each energizing a se arate applicator. Hence, only the applicator sections energized by the wave guides 21 and 22 will be described in greater detail; however, as can be seen from the illustrated embodiment, there are eight separate applicator sections, each heating a continuously moving cylindrical object, energized by the wave guides 1 l-l4. The individual applicator sections are stacked in two vertical columns of four, as seen in the plan view of FIG. 3.

The two applicator sections fed by the wave guides 21 and 22 are generally designated respectively by reference numerals 2S and 26, and each comprises a three-pass wave guide applicator wherein a continuation of its associated feed wave guide is formed into three folds about the broadwalls. Thus, the applicator section 25 includes a first wave guide section 27, a second wave guide section 28, and a third wave guide section 29. The wave guide sections 27 and 28 are connected by means of a U-shaped coupling 30, and the wave guide sections 28 and 29 are connected by a similar U-shaped coupling 31. A termination section 32 is connected to the downstream (relative to the direction of power flow) end of the applicator section 29. A lossy cylindrical object 33, supported by a semicylindrical conveyor belt 34 of low dielectric loss passes through the broadwalls of each of the applicator sections 27, 28 and 29 with the axis of the object 33 located approximately at the midpoint of the broadwalls of the applicator sections. The axis of the object 33 forms an angle of incidence, designated a, with the axis of the wave guide section. As used herein, the angle of incidence a is always meant to define the smaller of the two angles formed by the crossing of the axis of the cylindrical object and the axis of the wave guide in a plane generally parallel to the side walls of the applicator section.

v fully explained below.

The movement of the object 33 and conveyor 34 is in the direction of the arrows in FIG. 3, namely from top to bottom of the page. The angle of incidence, a, is illustrated for the sections 27 and 29 of the applicator in FIG. 3.

Metallic input and output ports for the applicator section 25 are designated respectively by reference numerals 36 and 37 (FIG. 3), and a water load including a conduit 38 of low dielectric constant through which water is continuously fed is provided in the termination section 32 for absorbing any power that may remain after the microwave energy leaves the final section 29. The axis of the conduit 38 preferably lies along the axis of the rectangular termination section 32, although it may meander in and out of the broadwalls.

Turning now to the applicator section 26, it too is a threepass applicator including first, second and third sections designated respectively 38, 39 and 40. The sections 38 and 39 are connected by means of a U-shaped coupling 41, and the sections 39 and 40 are connected by means of a second U- shaped coupling 42. The downstream end of the final section 40 is connected to a termination section 42 which is provided with a water load coupled through a conduit 43 similar to the conduit 38 for the termination section 32. A cylindrical object 44, held by a semi-cylindrical conveyor 45 is transported through the sections 38, 39 and 40. The axis of the object 44 is preferably parallel to the side walls (which are horizontal) of the sections 38, 39 and 40; and it forms an angle of incidence a with the axis of the rectangular sections 38, 39 and 40. Input and output conduits 47 and 48 are attached respectively to the outer broadwalls of the sections 38 and 40 at the inlet and exit apertures for the object 44 and the conveyor belt 45. As mentioned, the material used for the belts 34 and is preferably of a low dielectric loss factor, such as Mylar, so that the belt itself is not heated while passing through the applicator.

Turning now to FIG. 2, there is shown a transverse cross sectional view of the upper two applicator sections and a portion of the third applicator section of the right-hand column of sections as illustrated in FIG. 1. These applicator sections are generally designated respectively 26, 50 and 51. The cylindrical objects being treated in the applicator section 50 is designated 52, and the semi-cylindrical conveyor which supports it is designated 53. Similarly, a cylindrical lossy object 54 supported by a semi-cylindrical conveyor 55 is transported through the applicator section 51. The axis of each of the objects 52 and 54 forms an angle of incidence a with respect to the axis of the individual sections of the applicator 50 and 51 which is equal to the angle a formed by the axis of the cylindrical object 44 and the axis of its associated applicator sections, 38, 39 and 40.

For the three individual wave guide passes 38, 39 and comprising the applicator section 26, as viewed in FIG. 2, the broadwalls are shown in a vertical disposition whereas the side walls are horizontal. Power flows from the input feed section 11 through the elbow 15, the T 20, the wave guide 22, the first straight section 38, thence through the U-shaped coupling 41, the second straight section 39, the second U-shaped connector 42 and the third straight section 40, finally terminating in the section 42. Thus, as can be appreciated from FIG. 3, the power flows generally in the direction of movement of the object 44 in the sections 38 and 40 whereas for the section 39, power flows generally in a direction against the movement of the object 44 and its conveyor 45.

Referring again to FIG. 2, the section 38 is provided with an inlet aperture 38a and an outlet aperture 381:. The intermediate section 39 is also provided with an inlet aperture 39a which is alignedwith the aperture 39b, and an outlet aperture 38b. The final section 40 is provided with an inlet aperture 400 and an outlet aperture 40b (see FIG. 3) which feeds the exit conduit 48.

THEORY OF OPERATION Referring now to FIGS. 4 through 7, the theory and operation of the present invention will be described in greater detail. Referring first to FIG. 5, one section of a rectangular wave guide is shown in cross section, and it is generally designated by reference numeral 56. It includes upper and lower broadwalls 58 and 59, and first and second side walls 60 and 61. The cylindrical object being treated is designated by reference numeral-62. The length of the broadwalls, as measured between the interior surfaces of the side walls is designated a; and the corresponding length of the side walls is designated by letter b. The dimensions a and b may be constrained such that only the TE mode is allowed to propagate. That is the dimension a is everywhere between one-half A. (the wavelength of the excitation frequency) and a full A and the dimension b is everywhere less than or equal to one-half A These constraints may be summarized by the inequalities below:

The above restraints on the dimensions a and b for a given excitation frequency oppose an upper limit on the size of the wave guide.

The electric field intensity (E) for the TE mode in a plane transverse of the flow of power (that is, the plane of the page of FIG. 5) is a sinusoidal function, having a value of zero at each of the side walls 60 and 61 and a maximum value (5,) midway between the side walls. That is, referring to the graph in the lower portion of FIG. 5 wherein the abscissa represents the distance, x, from the side wall 60 to the side wall 61 and the ordinant is the intensity of the electric field, the line 65 represents the intensity across this transverse section of the wave guide. The intensity of the electric field is also schematically illustrated in the upper portion of FIG. 5 by the closeness of the electric field arrows 66 as they approach the center of the wave guide; but it will be noted that the electric field intensity vectors 66 extend perpendicularly between the broadwalls 58 and 59.

With the boundaries as defined in the illustration of FIG. 5, the equation for the electric field strength or intensity in the TE mode is given below as Equation (1). The x direction, as already mentioned, is the horizontal direction in FIG. 5, the y direction is the vertical direction, and the z direction is perpendicular to the plane of the page (that is, in the direction of flow of power).

The power transmitted through the wave guide in the TE mode is the integral of the scaler product of the Poynting vector and the differential cross sectional area of the guide (i.e. a X b). It is thus seen that the power distribution in a plane transverse of the direction of fiow of power is at a maximum along a plane midway between the side walls and parallel thereto for the TE mode. This is the plane of maximum field intensity; and it will be appreciated that the power may be attenuated along the direction of power flow if power is dissipated in any object within the wave guide. For higher order modes, namely TE, modes wherein m is an integer, there will be corresponding greater number of planes of maximum field intensity; but for rectangular wave guides, each plane will be coincident with the location of maximum field intensity and extend parallel between the side walls in the direction of power flow.

It can be seen from FIG. 5 that the cylindrical object 62 passes through the broadwalls 58 and 59 of the wave guide 57 with the axis of the object 62 lying substantially in the plane of maximum electric field intensity (which extends perpendicular to the plane of the page of FIG. 5 and along the dashed line x 0/2). Referring now to FIG. 4 wherein it is assumed that the microwave energy is propagating from the upper right hand comer to the lower left hand corner, the axis (designated by chain line 70) of the cylindrical object 62 defines an angle of incidence a with the axis (schematically shown by chain line 71) of the wave guide 57. As has already been mentioned,

the angle of incidence a is always taken to be the smaller of the two angles formed by the axis of the wave guide and the axis of the cylindrical object.

The setting of the angle a is best accomplished empirically because it depends upon many of the system parameters ineluding the dielectric constant and loss factor of the object being processed, the characteristic impedance of the wave guide, the water and salt content of the object in the case of Vienna sausage.

In order to appreciate the function of the setting of the angle, a, reference is made to FIGS. 6 and 7. In FIG. 6, the orientation of the axis 70 of the cylindrical object 62 is perpendicular to the plane of the broadwalls 58 and 59. This would be the ease for an angle of incidence, a, of 90. In this case, the electric field vectors extend everywhere perpendicular ,to the plane of page of FIG. 6; and these are designated by the crossed circles 73. In this case, it will be appreciated that the orientation of the electric field vector is everywhere parallel to the surface of the cylindrical object 62 so that for any arbitrary angle 0 taken in a plane transverse of the cylindrical object 72, the electric field density parallel to the exterior surface of the object is everywhere equal. This would accomplish a highly uniform surface heating of the object; however, it would also cause a maximum of reflection of the microwave energy back to the source because of the abrupt discontinuity in the field. Reflection of energy back to the source may be undesirable because it may cause significant heating within the magnetron tube (which preferably comprises the source of microwave energy) thereby reducing operating efficiency.

On the other hand, if the object 62 were oriented such that its axis 70 were colinear withthe axis of the wave guide, as diagrammatically illustrated in FIG. 7, then the electric field intensity vectors 73 would be perpendicular to the surface of the object along certain portions thereof (see the vector'73a); and at other locations, the electric field vector would be parallel to the surface (see the vector 73b). In this latter case, there would be a non-uniform heating of the surface of the object because the electric field density vector both parallel and perpendicular to the surface of the object would not be uniform per unit angle 0.

As explained in greater detail in my co-pending, co-owned application entitled WAVE GUIDE APPLICATOR AND METHOD, Ser. No. 817,097, filed Apr. 17, 1969, now US. Pat. No. 3,597,565, the amount of energy coupled into the surface of a lossy dielectric object depends upon the orientation of the electric field vector relative to that surface. If the electric field vector is perpendicular to the surface, coupling is at a minimum; whereas if the electric field vector is parallel to the surface of the object, coupling is a maximum. For all intermediate angular dispositions of the electric field vector, the amount of coupling may be broken down into its two component parts, one perpendicular to the surface and the other parallel to it. It will thus be appreciated from FIG. 7 that if a cylindrical object were transported along the axis of the wave guide, there would result a great disparity in heating of the object when one considers a circumferential path about the object.

The present invention orients the axis of the cylindrical object relative to the axis of the wave guide at an intermediate angle of incidence, a, preferably within the range of to 80 and depending upon the system parameters, to reduce reflection while increasing the coupling of energy into the surface of the object and, at the same time, to make the heating of the object more uniform about the surface of the object.

By thus forming an angle of incidence between the axis of the object being treated and the axis of the wave guide in the rage of 15 to 80 and preferably in the range of to 50, I have been able to reduce reflection of microwave energy back to the source while causing the object to experience a more uniform surface heating. It will be appreciated that even for that portion of the electric field which strikes at the vertical center of the object, there is a component which is parallel to the surface because the orientation of the object. That is, even for that portion of the electric field which strikes a surface of the object in line with its axis (corresponding to the electric field vector 73a in FIG. 7, there is a component of that vector which is parallel to the surface of the object because the oh- 5 ject is not passing perpendicular to the plane of the page of FIG. 7 in the inventive system but rather passes at an angle, a, relative to a line perpendicular to the plane of the page.

EXAMPLE Using the inventive system, Vienna sausage was passed through a WR 975 wave guide system involving two passes over the product. The input power was 16 KW; and the frequency of operation was 915 MHZ. The angle a was set at 26; and the sausage was conveyed at 50 feet per minute. The power absorbed was on the order of 75 percent while the reflected power was less than 1 KW. Upon observation, it was determined that the sausage was uniformly heated and the mixture congealed. Further increase in efficiency can be expected by'the three-pass applicator of the illustrated embodiment.

As a second example, a strand of Vienna emulsion was put through a meander wave guide at an angle of 18 relative to the axis of the wave guide and a power level of 8 KW. The speed of the cylindrical emulsion was approximately 20 feet per minute. Power measurements indicated that approximately 65 percent of the incident power was being absorbed by the object. The angle of the Vienna through the wave guide was then changed to 26. At 8 KW. input power, the Vienna was conveyed through the applicator at 25 feet per minute. Power measurements indicated approximately 75 percent of the microwave energy was being absorbed at an angle of incidence of 26. Microwave power and conveying speed were increased until a speed of 50 feet per minute were reached. At this speed, a power level of 15 to 16 KW. was used. Effective heating time for the Vienna was 2.4 seconds; and from a visual inspection, the results were good.

It may be desirable in some instances to taper the broadwalls of the wave guide in the applicator section to cause the distribution of field intensity to become more concentrated; and it is one of the advantages of the present invention that despite such changes in the geometry of the guide or changes in the diameter of the product being treated that such changes will not substantially affect the uniform heating of the object.

Other equivalent wave guide applicators may be used in place of the rectangular cross section for example circular, eliptical and ridged wave guide wherein the field vectors extend between opposing interior wall portions and are generally parallel to a line transverse to the axis of the guide.

The axial displacement of the entrance and exit apertures through which the object is conveyed have been found to be advantageous in that this displacement causes less discontinuity in the EM field than if they were both located in a plane perpendicular to the direction of power flow.

Having thus described in detail an illustrated embodiment of the present invention together with an explanation of the theoretical aspects thereof, persons skilled'in the art will be able to modify certain of the structure illustrated and to substitute equivalent elements for those which have been disclosed while continuing to practice the inventive principle; and it is therefore intended that all such modifications and substitutions be covered as they are embraced within the spirit and scope of the appended claims.

I claim 1. A system for applying microwave energy to a lossy cylindrical object defining an axis comprising: a source of microwave energy; microwave applicator means excited by said source to generate an electric field wherein the field lines are substantially parallel and have a maximum intensity along a given plane; and support means carrying said object for transporting the same through said field such that the axis of said object be generally in said plane and forms an acute angle in the range of about 10 to with said field lines whereby said field lines incident on the surface of said object are inclined relative thereto over substantially the entire surface of said object.

2. The system of claim 1 wherein said applicator means includes a wave guide of rectangular cross section including a pair of opposing broadwalls and a pair of opposing side walls and wherein said source means excite said applicator means in the TE mode where m is an integer.

3. The system of claim 2 wherein said source excites said applicator in the TE mode and wherein said electric field lines extend perpendicularly between said broadwalls and the plane of maximum field intensity extends along the direction of power flow and midway between said side walls.

4. The system of claim 3 wherein the axis of said object forms an acute angle of about 26 with the axis of said rectangular wave guide.

5. The system of claim 1 further comprising a termination section downstream of said applicator section for terminating the same; and a load in said termination section for absorbing all remaining power without reflection.

6. The system of claim 1 wherein said angle of incidence is selected to minimize the reflection of energy back to said source while maintaining a more uniform heating of the surface of said object.

7. The system of claim 1 wherein said applicator means comprises a rectangular wave guide excited in the TE mode and folded about one broadwall whereby said object may be passed through two separate sections of the same applicator at the same angle of incidence.

8. The system of claim 1 wherein said applicator means comprises a rectangular wave guide excited in the TE mode and twice folded about one of the broadwalls thereof to define three separate applicator sections whereby said cylindrical object may be passed successively through said sections.

9. The system of claim 8 further comprising termination means connected to said wave guide downstream of said applicator for absorbing substantially all of the power not absorbed by said object.

10. A method for heating a cylindrical object defining an axis comprising exciting a hollow microwave applicator to generate an electric field wherein the field lines are substantially parallel and have a maximum intensity along a given plane, and passing said object through said fields in an orientation such that the axis of the object lies generally in the plane of maximum field intensity and fonns an acute angle in the range of about 15 to with said field lines.

11. In a method of applying microwave energy to a cylindrical object the step of passing said object through the transverse centers of the broadwalls of a rectangular wave guide excited in the TE mode with the axis of the object forming an angle in the range of 15 to 80 with a line extending perpendicular to said broadwalls.

12. In apparatus for heating an elongated object having a curved exterior surface, the combination of a microwave applicator generating an electric field having a substantially uniform orientation and means passing said object through said field in the direction of elongation thereof and in an orientation such that said field vectors form an angle relative to the surface of said object of less than about substantially over said entire curved surface thereof.

13. In apparatus for heating an elongated object having a curved exterior surface, the combination of an elongated microwave applicator defining an axis, source means exciting said applicator with an electric field having a substantially uniform orientation transverse of said applicator axis, and means passing said object into said applicator at one location and out of said applicator at a second location spaced axially of said first location.

IF 4 k 

1. A system for applying microwave energy to a lossy cylindrical object defining an axis comprising: a source of microwave energy; microwave applicator means excited by said source to generate an electric field wherein the field lines are substantially parallel and have a maximum intensity along a given plane; and support means carrying said object for transporting the same through said field such that the axis of said object be generally in said plane and forms an acute angle in the range of about 10* to 80* with said field lines whereby said field lines incident on the surface of said object are inclined relative thereto over substantially the entire surface of said object.
 2. The system of claim 1 wherein said applicator means includes a wave guide of rectangular cross section including a pair of opposing broadwalls and a pair of opposing side walls and wherein said source means excite said applicator means in the TEmo mode where m is an integer.
 3. The system of claim 2 wherein said source excites sAid applicator in the TE10 mode and wherein said electric field lines extend perpendicularly between said broadwalls and the plane of maximum field intensity extends along the direction of power flow and midway between said side walls.
 4. The system of claim 3 wherein the axis of said object forms an acute angle of about 26* with the axis of said rectangular wave guide.
 5. The system of claim 1 further comprising a termination section downstream of said applicator section for terminating the same; and a load in said termination section for absorbing all remaining power without reflection.
 6. The system of claim 1 wherein said angle of incidence is selected to minimize the reflection of energy back to said source while maintaining a more uniform heating of the surface of said object.
 7. The system of claim 1 wherein said applicator means comprises a rectangular wave guide excited in the TE10 mode and folded about one broadwall whereby said object may be passed through two separate sections of the same applicator at the same angle of incidence.
 8. The system of claim 1 wherein said applicator means comprises a rectangular wave guide excited in the TE10 mode and twice folded about one of the broadwalls thereof to define three separate applicator sections whereby said cylindrical object may be passed successively through said sections.
 9. The system of claim 8 further comprising termination means connected to said wave guide downstream of said applicator for absorbing substantially all of the power not absorbed by said object.
 10. A method for heating a cylindrical object defining an axis comprising exciting a hollow microwave applicator to generate an electric field wherein the field lines are substantially parallel and have a maximum intensity along a given plane, and passing said object through said fields in an orientation such that the axis of the object lies generally in the plane of maximum field intensity and forms an acute angle in the range of about 15* to 80* with said field lines.
 11. In a method of applying microwave energy to a cylindrical object the step of passing said object through the transverse centers of the broadwalls of a rectangular wave guide excited in the TE10 mode with the axis of the object forming an angle in the range of 15* to 80* with a line extending perpendicular to said broadwalls.
 12. In apparatus for heating an elongated object having a curved exterior surface, the combination of a microwave applicator generating an electric field having a substantially uniform orientation and means passing said object through said field in the direction of elongation thereof and in an orientation such that said field vectors form an angle relative to the surface of said object of less than about 90* substantially over said entire curved surface thereof.
 13. In apparatus for heating an elongated object having a curved exterior surface, the combination of an elongated microwave applicator defining an axis, source means exciting said applicator with an electric field having a substantially uniform orientation transverse of said applicator axis, and means passing said object into said applicator at one location and out of said applicator at a second location spaced axially of said first location. 